#!/usr/bin/perl --

((@ARGV ge 1) && (@ARGV le 3)) || die "Usage: string1 string2 [target_file]\n";

$exchange1 = shift;
$exchange2 = shift;
$target    = shift;

if (defined($target)) {
  open(T,$target) || die "Cannot open $target: $!\n";
  $tmp_id = $$;
  $tmp_file = join("",$tmp_id,".srp");
	open(C,">$tmp_file") || die "Cannot open $tmp_file: $!\n";
	while(<T>){
	$_ =~ s/$exchange1/$exchange2/g;
	print C;
        }
  close(T); close(C);
  system "cp $tmp_file $target";
  system "rm $tmp_file";
  }
  else {
      @files = <*>;
      foreach $file (@files) {
                     open(T,$file) || die "Cannot open $file: $!\n";
                     $tmp_id = $$;
                     $tmp_file = join("",$tmp_id,".srp");
	             open(C,">$tmp_file") || die "Cannot open $tmp_file: $!\n";
	             while(<T>){
	             $_ =~ s/$exchange1/$exchange2/g;
	             print C;
		     }
                 close(T); close(C);
                 system "cp $tmp_file $file";
                 system "rm $tmp_file";
		 }
  }